1. Field of the Invention
The present invention relates to a digital signal transmission apparatus and a method for transmitting a digital signal, which are suitably used to transmit a video signal between, for example, a camera and a camera control unit (CCU), to a digital signal receiving apparatus and a method for receiving a digital signal, and to a digital signal transmission system.
2. Description of the Related Art
In the related art, when a plurality of cameras are used, for example, in drama shooting in a studio or in shooting a baseball game, a plurality of cameras are placed at different places in order to capture each necessary scene. The plurality of cameras are usually centrally controlled at one place.
Each camera is connected to an individual camera control unit (CCU) that controls the camera and that supplies electrical power to the camera. The reason for this is that, if a power supply is secured at one place, it is possible to supply electrical power to each camera. Another reason is that centralized control is more convenient because an input to a switcher that receives signals from a plurality of CCUs and that selectively outputs a predetermined signal becomes easier by applying synchronization without phase difference to individual cameras.
A video signal is transmitted mutually between a camera and a CCU. That is, a captured-image video signal is sent as a main line signal from the camera to the CCU, and conversely, a return video signal for confirming a final main line video that is recorded by a cameraman or that is on-air is sent from the CCU to the camera.
In the related art, for bidirectional video signal transmission between a camera and a CCU, when broadly classified, there are three types of method described below.
(1) There is bidirectional video signal transmission that involves transmission using an optical fiber cable. Since video transmission between a camera and a CCU is bidirectional transmission, it is common practice that two optical fiber cables, one for the upstream and one for the downstream, are used to perform digital transmission. That is, one optical fiber cable is used to send a main line signal from the camera to the CCU, and the other optical fiber cable is used to send a return video signal from the CCU to the camera.
FIG. 6 shows a transmission system 200A using an optical fiber cable. The transmission system 200A is configured in such a manner that a camera 201 and a CCU 202 are connected to each other with an optical fiber cable 203. A remote controller 204 and monitors 205 to 207 are connected to the CCU 202.
(2) There is bidirectional video signal transmission that involves transmission using a type of coaxial cable called a triaxial cable. This is a method in which a double-shielded wire of a triaxial cable is used, signals in both directions, including that for the power supply, are modulated, and frequency multiplexing transmission is performed. A transmission system using a triaxial cable in this manner is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 1998-341357.
FIG. 7 shows an analog transmission system 200B using a triaxial cable. The transmission system 200B is configured in such a manner that a camera 201 and a CCU 202 are connected to each other with a triaxial cable 208.
In this case, a digital video signal as a main line signal that is output from the camera 201 or a return video signal output from the CCU 202 are each converted into an analog video signal by using a D/A converter, thereafter FM modulation or AM modulation is performed thereon, and bidirectional transmission is performed by means of a frequency multiplexing method using a multiplex filter. On the reception side, the analog video signal that is obtained after FM demodulation or AM demodulation is performed once thereon is A/D-converted into a digital video signal and is output.
FIG. 8 shows an example of the circuit configuration on the camera 201 side and on the CCU 202 side. On the camera 201 side, digital video signals as main line signals, that is, a luminance signal Y that is 10-bit parallel data and color difference signals Cb/Cr that are 10-bit parallel data, are each converted from a digital signal into an analog signal by D/A converters 210 and 211. Thereafter, the signals are AM-modulated by AM modulators 212 and 213, and the signals are combined by a multiplex filter 214 and are sent to a triaxial cable 208. The color difference signals Cb/Cr indicate sequence signals of a blue color difference signal Cb and a red color difference signal Cr.
In connection with the above, on the CCU 202 side, the AM-modulated signals of the luminance signal Y and the color difference signals Cb/Cr are separated and extracted from the triaxial cable 208 by a multiplex filter 215. Thereafter, the AM-modulated signals are demodulated by AM demodulators 216 and 217, the AM-modulated signals are converted from analog signals into digital signals by A/D converters 218 and 219 in order to obtain a luminance signal Y and color difference signals Cb/Cr, which are 10-bit parallel data.
On the CCU 202 side, a return video signal Ret. Video (luminance signal Y) that is 10-bit parallel data is converted from a digital signal into an analog signal by a D/A converter 220. Thereafter, the signal is FM-modulated by an FM modulator 221, and the signal is sent to the triaxial cable 208 through the multiplex filter 215.
In connection with the above, on the camera 201 side, an FM-modulated signal of the return video signal Ret. Video is extracted from the triaxial cable 208 by the multiplex filter 214. Thereafter, the FM-modulated signal is demodulated by an FM demodulator 222, the signal is converted from an analog signal into a digital signal by the conversion, and a return video signal Ret. Video (Y) is obtained by an A/D converter 223.
FIG. 9 shows an example of the arrangement of frequencies of an analog transmission system using a triaxial cable. The center frequency of the AM-modulated signals (the color difference signals Cb/Cr) transmitted from the camera 201 to the CCU 202 is set at 21.6 MHz. The center frequency of the AM-modulated signal (the luminance signal Y) transmitted from the camera 201 to the CCU 202 is set at 64.8 MHz. The center frequency of the FM-modulated signal (the luminance signal Y) transmitted from the CCU 202 to the camera 201 is set at 126 MH. As a result, the frequency bands are made different.
Although not shown in the example of the circuit configuration of FIG. 8, an audio signal and a control signal, which are frequency-multiplexed, are transmitted between the camera 201 and the CCU 202. FIG. 9 also shows an audio signal SA1 transmitted from the camera 201 to the CCU 202, and an audio signal SA2 transmitted from the CCU 202 to the camera 201.
(3) As a system for bidirectionally transmitting digital video by using a triaxial cable, there is a transmission system using a time-domain multiplexing method. That is, a time period is divided so that a time period during which a main-line video signal is sent and a time period during which a return video signal is sent are alternately determined, and by repeating this operation, bidirectional transmission is performed. In this system, management of time is difficult. Furthermore, as the transmission distance becomes longer, the wait time for the sent video signal to be reached becomes longer, and the time period for sending a video signal becomes shorter.